Scientific notation or separators?
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Scientists generally write large numbers using powers of 10. For example, France's national debt is written not as €2,332,000,000,000 but as €2,332 × 109. This saves the tedious task of counting the zeros! Such notation is useful only when the number ends in many zeros, as is the case for a value corresponding to a known physical quantity with only a few significant digits. Should the speed of light be written as 299,792,458 m/s or 2.99792458 × 108 m/s? Scientific notation quickly gives us an idea of the order of magnitude. Traditional notation, however, is the easiest to say aloud.
In prose, writers are reluctant to use digits to write a number, a fortiori notation involving powers of 10. A mixed system is preferred, expressing the debt as "2,332 billion euros." The situation is different when dealing with an arbitrary number—for example, France's debt to the nearest euro at time t (see dettepublique.fr), which stood at €2,332,798,169,265 when this article was written. It is impossible to read this quickly unless you already know its approximate value.
To avoid these pitfalls, it has become customary to separate such numbers into groups. They are generally grouped in threes, matching the way numbers are named in most Western languages. In France, a space is used, so 2,332 billion 798 million 169 thousand 265 is written 2 332 798 169 265. Unfortunately, there is no universal separator. The British prefer a comma, the Germans a full stop (or sometimes a space), and German-speaking Swiss people generally use an apostrophe…
Indian separators
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